Osmium and Iridium are the two most dense elements in pure form. They are also very pretty metals, though osmium tarnishes to a toxic oxide and should therefore be handled carefully. In powdered form it is actually quite dangerous, but solid lumps can be handled with care.

Which is the most dense element, osmium or iridium? That honor has changed hands a couple of times over the years. How, you might ask? Because measuring the density of a pure element isn't nearly as straightforward as you might think. For one thing, it can be different for different crystalline forms (allotropes) of the same element. For example, diamond is much denser than graphite: Which is the true density of carbon? Both. In the case of metals, it may be impossible to grow single crystals, so you're measuring the density of a polycrystalline mixture, and the density may depend on details of how the material cooled. Minute impurities can also have a big impact on the density.

So, as measurements and purities were refined over the years, the current best values of osmium and iridium kept changing, and that's how they traded places as most dense of all. I solved this dilemma by only having room to engrave three decimals places of the density, and to three decimal places their density is identical, so I didn't have to pick sides.

Antique phonograph needle.
People playing antique phonograph cylinders (the kind that are played acoustically with no electronics between the needle and the speaker) care a lot about their needles. Probably this is because every time they play a cylinder, an irreplaceable historical artifact is irreparably damaged a little bit more, how much depending on what needle you're using.
Fortunately, some people are transferring and archiving the recordings. The sound for this sample is a short excerpt of a wax cylinder recording taken from the website of a person who offers to transfer wax recordings to CD in exchange for a copy of them.
In any case, the preferred material for needles is of course diamond, with sapphire a good second. But, as was very nicely explained in the letter that came with this fine needle, if you're going to use a metal needle to get a certain kind of tone, an osmium one will last much longer, and leave fewer shards of metal stuck in the grooves, than will a steel needle.
My microscopic examination reveals no joints in the metal, and Randall Anderson, the source, explains as follows:

"Yes they are all osmium. The tungsten ones usually have just a tip, or a central shaft set in brass, but the osmium ones are usually all the way. The thicker the needle the louder the tone, thus they come in soft, medium, and loud tone. The last of the manufacturers went out of business in the late 1970's. I bought out their final inventory. Only NOS (New Old Stock) exist at this point."

On the other hand, the needle is quite unmistakably attracted to a magnet, which it should not be if it were solid osmium. It might be plated, or it might be one of several osmium/iron alloys that are discussed in connection with phonograph needles.

Preliminary analysis by x-ray fluorescence spectroscopy at the Center for Microanalysis of Materials, University of Illinois (partially supported by the U.S. Department of Energy under grant DEFG02-91-ER45439) indicates that there is absolutely no osmium in this needle. Not the slightest ghost of a trace. But, this is a surface analysis only and it's possible the machine was missing the tip of the needle, so some hope remains: I plan further testing after I learn more about the x-ray beam's exact location within the sample area of the machine. What the machine did see was 60% iron, 40% nickel.

Another antique phonograph needle.
This is a different brand, one that claims to have an osmium tip welded on it (see below for packaging making this claim). It's still magnetic, though without breaking off the tip it's a bit hard to tell whether the tip itself is, or whether it's just the rest of the shaft that is attracted to the magnet.

Preliminary analysis by x-ray fluorescence spectroscopy at the Center for Microanalysis of Materials, University of Illinois (partially supported by the U.S. Department of Energy under grant DEFG02-91-ER45439) indicates that there is absolutely no osmium in this needle either. Again, it could be that the osmium is only in the tip, which I may have missed, or that a plating obscured it. In this case the machine saw 57% iron, 43% copper.

Still more antique phonograph needles, these in original packaging.
After he sent it but before I knew about it, I won an auction for five more osmium needles identical to the one George had sent me (see above). I photographed his and kept this set in its original packaging, though by the time you read this I may have traded one for a selenium rectifier.Source:eBay seller startgrooveContributor:eBay seller startgrooveAcquired:31 August, 2002Price: $17Size: 0.5"Purity: <50%

Yet a third brand of osmium phonograph needle.
Despite striking out with two different brands of "osmium" needle (no actual osmium), I remain eternally optimistic that some day I'll find one that contains real osmium. This is a brand I haven't seen before, so maybe....
I have not tested it yet, but before too long I'll know if my hopes will once again be dashed against the rocks of eBay.Source:eBay seller 00018Contributor:Theodore GrayAcquired:4 January, 2003Price: $10Size: 0.2"Purity: <20%

Real osmium.
Osmium metal is hard to get, and very expensive. In fact, it was in the very last group of elements I was able to acquire to complete my collection. It was donated by the extremely kind Max Whitby of the The Red Green & Blue Company, which sells a complete collection of elements (including this one).

Osmium and Iridium are the two densest elements in the world (they are in fact so close in density that which one is considered the densest has switched a couple of times over the years). Even though this is a quite small lump, you can feel its weight when you shake the bottle: Quite surprising. Having a large block of this would be remarkable, but the closest I'm likely to come to that is my large blocks and cylinders of tungsten, which are only about 15% less dense.

Sample from the Everest Set.
Up until the early 1990's a company in Russia sold a periodic table collection with element samples. At some point their American distributor sold off the remaining stock to a man who is now selling them on eBay. The samples (except gases) weigh about 0.25 grams each, and the whole set comes in a very nice wooden box with a printed periodic table in the lid.

Museum-grade sample.
In early 2004 Max Whitby and I started selling individual element samples identical or similar to the samples we use in the museum displays we build. These are top-quality samples presented in attractive forms appropriate to the particular element. They are for sale from Max's website and also on eBay where you will find an ever-changing selection of samples (click the link to see the current listings).

This bottle contains about 50 grams of arc-melted buttons made in Max's reduced-pressure argon-arc furnace.

I chose this sample to represent its element in my Photographic Periodic Table Poster. The sample photograph includes text exactly as it appears in the poster, which you are encouraged to buy a copy of.

Osmiridium (aka Iridosmine).
These are very small granules of native (naturally occurring) osmium-iridium alloy. These two metals are often found combined this way, and because the alloy is actually in many ways more useful than either element on its own, the mixture is often used just as it's found, for example in the tips of expensive fountain pens. Don't be fooled by the picture: This stuff is about the size of fine sand, I've just got a really good macro lens.Source:eBay seller entropydaveContributor:Theodore GrayAcquired:15 February, 2006Text Updated:5 December, 2006Price: $85Size: 0.02"Composition:IrOs

Big bag of beads. (External Sample)
This is a large ($18,000) order of platinum group metals placed by an excellent customer of my partner Max Whitby's element sales business. I happened to be visiting Max in London just before the order needed to be shipped to our customer in the US, so I hand-carried the precious cargo home rather than risking international shipping. These beads were made by Max in his reduced pressure argon arc furnace.

Big bag of beads. (External Sample)
This is part of a large ($18,000) order of platinum group metals placed by an excellent customer of my partner Max Whitby's element sales business. I happened to be visiting Max in London just before the order needed to be shipped to our customer in the US, so I hand-carried the precious cargo home rather than risking international shipping. These beads are made by Max in his reduced pressure argon arc furnace.

The order consisted of equal volumes of ruthenium, rhenium, osmium, and iridium. Here is what the whole collection looks like:

The customer wishes to remain anonymous, so you'll just have to keep wondering where this remarkable trove of rare metals currently resides: The only thing you can be sure of is that I don't have it.Location: AnonymousPhotographed:4 September, 2007Text Updated:6 September, 2007Size: 0.25"Purity: 99.99%